A conventional well pumping system includes a large rocker arm for reciprocating a pump rod which extends downhole for connection to a piston of a pump mounted within the well. The rocker arm typically includes a pivotally mounted "walking beam" and "horsehead" mounted on a framework adjacent the well head. The walking beam pivots to reciprocate the pump rod vertically. The walking beam is commonly driven by a complex mechanical drive system. One such drive system can include a crank connected between the walking beam and a rotating arm mounted on a drive shaft driven through a gear box from a drive motor.
It often becomes necessary, or at least desirable, to make mechanical changes to the pump drive system dynamics during use. For instance, changing the stroke length or stroke rate (strokes per minute) of the pump often requires mechanical changes which are time consuming and costly. To change the stroke length, for example, requires changing the pivot pin location on the walking beam, together with other mechanical changes in the linkage between the walking beam and the downhole pump. These changes can require special equipment and additional personnel. It can require a crane to lift the walking beam while the beam's pivot is changed, for example. At least a half day's production time can be lost when changing the stroke length and stroke rate of the pump.
Prior well pumping systems also commonly experience field conditions that produce wear and tear on the equipment and reduce operating efficiency. Substantial loads are imposed on the pump rod of conventional pumping equipment. Large shock loads, especially, are placed on the pump rod as it reciprocates in a well which can be several thousand feet deep, or more. Downhole conditions in the well are often unpredictable and can cause sudden movements or directional changes in the pumping equipment.
Wear and tear on conventional well pumping equipment is especially severe when the pump undergoes a pumping-off condition, in which lift occurs above the fluid level in the well. This condition pulls a vacuum in the production tubing and creates severe impacts on the pumping equipment if the condition is not corrected. In prior well pumping systems, a pumping-off condition is sensed and the pump is stopped. Often, steam is injected downhole to change the viscosity and flow rate of the oil in order to correct the condition.
The present invention provides a system for automatically controlling the motion of a rocker arm-driven well pump. The control system senses the actual motion of the rocker arm throughout its pumping cycle and constantly adjusts its travel in accordance with a desired pumping motion. The control system provides a number of improvements over the conventional mechanically operated well pumping equipment. For instance, the stroke length and number of strokes per minute of the rocker arm can be easily adjusted Acceleration and deceleration of the walking beam can be controlled for each upstroke independently of each downstroke of the beam. These controls are equivalent to moving the pivot of the fulcrum of a conventional pump; but such control is produced without requiring complex mechanical changes to the pumping equipment. Precise control over pumping motion throughout the pumping cycle also reduces shock loading and wear and tear on the equipment. In addition, the control system can pre-sense a pumping-off condition and quickly adjust the stroke length to maintain production while avoiding impact loading on the equipment. Thus, wear and tear on the equipment are reduced, and valuable production time is not lost.